Electronic switching system



Dec. 8, 1964 P. E. OSBORN ELECTRONIC swncamc SYSTEM 11 Sheets-Sheet 1 Filed May 2, 1961 N3 E m qomhzoo x5 4 on 95 3205 mic N42 zoam mz t INVENTOR. Peter E. Osborn Affy.

Dec. 8, 1964 P. E. OSBORN 3,160,712

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet 4 SEQUENCE REGISTER CONTROL INVENTOR. Peter E. Osborn FIG. 4

Dec. 8, 1964 P. E. OSBORN 3,160,712

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet 6 I44 I48 I46 I52 I45 154 1i I56 ;I88 A CR5 A2 CP 1.5 A A3 CPI.5

---o I Lei- H GH MAGNETIC CP.5 o SPE'ED gE DIS7'. MAGNET/C MEMORY 6'6 DIST: I50

DP4O 1 CP.5 7 I CP-IAQ HIGH CPIB SPEED W PULSE SOURCES FIG 6 6I0 o-MJ FIG. I3

FIG. 2 FIG.3

FIG. 4 FIG. 5 HIGH SPEED CLOCK FIG.

FIG. 14

FIG. I0 FIGJI TIME IN MICROSECONDS INVENTOR. FIG Peter E. Osborn Dec. 8, 1964 P. E. OSBORN 3,160,712

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet '7 INVENTOR. Peter E. Osborn Dec. 8, 1964 P. E. OSBORN 3,160,712

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet a JNVEN TOR.

Pefer E. Osborn Dec. 8, 1964 P. E. OSBORN 3,160,712

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet 9 1N VEN TOR. Peter E. Osborn Dec. 8, 1964 P. E. OSBORN ELECTRONIC SWITCHING SYSTEM 11 Sheets-Sheet 10 Filed May 2, 1961 vw mm mm m 0m 2 2 i E Q E E S I LLLL LLLL Affy.

1964 P. E. OSBORN 3, ,7

ELECTRONIC SWITCHING SYSTEM Filed May 2, 1961 11 Sheets-Sheet 11 m LL E c a [L 3 IE INVENTOR.

: Peter E. Osborn u 2 a; .f A

AHy.

United States Patent G 3,160,712 ELEC'ERQNZC SWTlCl-IEQG SYSTEM Peter E. Osborn, Addison, Iii, assignor .to Automatic Electric Laboratories, inc, Northlake, lit, a corporation of Deiaware Filed May 2, 1961, Ser. IIo. 107,213 (Dims. (E5. 179-18) This invention relates to an electronic switching system, and more particularly to a communication system using time division multiplex principles for switching.

A communication system of the type described above is disclosed in the copending US. patent application to A. H. Faulkner et a1. Serial No. 843,380, filed September 30, 1959, now Patent No. 3,015,699. In that invention, connections are selectively established between any of a group of line circuits and any of a group of connecting units by providing common control equipment shared on a time division multiplex basis by the connecting units and by providing in said common control equipment logic circuits for use in selecting the individual line circut to be effectively connected with an individual connecting unit in a given call and also circuits for selectively operating the switching arrangement to establish the connections.

The common control equipment comprises two groups of time division multiplex circuits having different distribution cycles; one of the groups of circuits being associated with high-speed storage apparatus including a highspeed memory and circuits for storing, on a time sharing basis, the identity of the line circuit to be effectively con nected with an individual connecting unit in a given call, and operably at the hgh-speeds necessary for time division multiplex transmission of voice frequency signals; and the other group, because of the comparatively low-speed of the conventional dial used at the subscriber substations, being associated with low-speed storage apparatus including a low-speed memory and circuit for storing, on a time sharing basis, the state of said call in accordance with supervisory signals received from the line circuits.

It is the object of this invention to provide new and improved arrangements for controlling the selection and establishment of connections in an electronic switching system.

A feature of the invention is the provision of signaling means at each of the subscriber substations for providing, at a speed which is compatible with time division multiplex operation, variable-width direct-current supervisory signals.

Another feature of the invention relates to the registration of the digits represented by the variable-width directcurrent supervisory signal. According to this feature, the register is advanced with every other multiplex pulse thus variations in the operation of the signaling means can be tolerated without faulty digit registration. If an attempt were made to advance with every pulse there could be no tolerance since the signal from the substation can start at any random time.

Another feature of this invention relates to the common control equipment. Two groups of time division multiplex circuits, each having the same distribution cycle, are used. One group of circuits comprises storage apparatus having circuits for storing, on a time sharing basis, the identity of the line circuit to be effectively connected with an individual connectng unit in a given call, and the other group of circuits comprises storage apparatus having circuits for storing, on a time sharing basis, the state of said call in accordance with supervisory signals received from the line circuits.

A further feature of this invention is the provision of a single memory which constitutes a portion of the storage ice apparatus of both of the above-mentioned two groups of time division multiplex circuits.

The above-mentioned and other objects and features of this invention and the manner of attaining them will become apparent and the invention itself will be understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings comprising FIGS. 114 wherein:

FIG. 1 is a single line block diagram of the entire system, with the single ferrite-core memory shown schematically;

FIGS. 2 to 6, when arranged together as shown in FIG. 13, comprises a diagram showing the interconnections of the various units, with the subscriber substations and line circuits shown by schematic and block diagrams in FIG. 2, transmission circuits shown by block diagrams in FIG. 3, the register control circuits shown by block diagrams in FIG. 4, the high-speed registers shown by block diagrams in FIG. 5, and the pulse sources shown by block diagrams in FIG. 6;

FIG. 7 is a functional block diagram of the timer;

FIG. 8 is a functional block diagram of the sequence circuit;

FIG. 9 is a functional block diagram of the line number advance circuit, and the busy circuit;

FIGS. 10 and 11, arranged as shown in FIG. 14, is a graph showing the sequence of operation of the various units;

FIG. 12 is a graph showing the output pulses of the high-speed clock;

FIGS. 13 and 14 show how various ones of the figures are to be arranged.

GENERAL DESCRIPTION (FIG. 1)

Referring to FIG. 1, the exchange includes line circuits LCll to LCM associated with stations S11 t0 S00 and a plurality of link circuits LKl to LK20 interconnected by a time division multiplex transmission highway MLl-MLZ having a transmission control unit interposed therein. Any two line circuits may be effectively connected through any link circuit by selectively supplying control pulses to them.

The signaling circuits 170 supply supervisory tones which are transmitted by time division multiplex over line 172 and the highway MLl-MLZ to the line circuits of calling lines. The signaling circuits 176 also supply ringing control signals over conductors in the control line 134 to the line circuits of called lines.

The function of remembering which circuits are interconnected on a time division multiplex basis over highway MLl-MLZ and of supplying control pulses to the selected transmission gates in the appropriate time slots is performed by a high-speed memory 150 in conjunction with a line numberregister and a signal state register 1400. The control pulses are supplied to the line circuits over line 134, to the link circuits over lines DP and S-T, and-to the signaling circuits over line 162.

The selective registration in the high-speed memory through the line number register 13d and the signal state register 14% is controlled by the register control circuits 4%. These circtu'ts 401') control .the line-finding function of scanning to find a line which has initiated a call and causing a connection to be established to the calling line; and the connector function of detecting dial pulses from the calling line and causing a connection to be established to the called line. These control circuits 400 are shared by all of the links on a time division basis, using the high-speed memory for storage.

The pulse sources 60!) comprise pulse generators and distributors for supplying all of the pulses required by the exchange.

'state should be changed.

The high-speed memory 158 comprises a coordinate array of ferrite-cores. The horizontal rows are associated with the links, and the vertical columns are associated with flip-flop type storage devices in the associatedunits 400, 130, and 1400. A pulse distributor in the pulse sources 609 supplies pulses to the horizontal conductors in turn. Each horizontal row has a read winding and a /-write winding threaded through the cores of the row, and each vertical column has a sense winding and a /2- write winding. During each stage of the distributor, a read pulse is supplied through the read winding of the row, causing the state of each core of the row to be transferred by means of the sense windings to the flip-flops. The information in the flip-flops is then utilized and possibly altered .by the associated circuitry. A /z-write pulse is applied to the horizontal winding, and coincidently to selected ones of the vertical windings to return the information from the flip-flops to the cores. This is repeated, in turn, for each horizontal row during successive stages of the distributor. The high-speed memory comprises cores in three columns H, K, and L for registering the timer states, three columns M, N, and P for registering the sequence state, two columns X and Y for registering the line number advance states, one column B for registering the busy states, five-columns TC to T6 for registering the tens digit, five columns UC to US for registering the units digit, and five columns BT, DT, RG, RT, and ST for registering the signal states. Each horizontal row is associated with one time slot of the multiplex transmission. Each stage of the distributor comprises a 0.5-microsecond read pulse followed by a 1.5-microsecond /2-write pulse in a 2-microseconds time slot. The horizontal /z-write windings are connected at one end to the distributor and at the other end over line DP to the link circuits for transmission control} The /z-Write windings of the first two rows are connected over leads DPl and DPZ to link 1 to control the calling party and called party transmission, respectively. Successive pairs of horizontal rows are in like manner coupled to successive links, so that each link is permanently associated with two high-speed memory rows corresponding to two transmission time slots, one for the calling party and the other for the called party. Also, during each of the 1.5-microseconds pulse intervals, the line number register 130 translates the two-out-of-five code registration of the tens digit to a one-out-of-ten code signal supplied to a conductor in line 134, and the two-out-of-five code of the units digit is translated to a one-out-of-ten code signal supplied to another conductor of line 134, to control the transmission in the line circuit corresponding to this number. At the same time, the signal state register 1400 controls the transmission of supervisory tones to the calling line, ringing to the called line, and switc through of the link transmission'gates, as required.

The register control circuit 40d receives hookswitch and line-busy information from the line circuits and registers this information in flip-fiops for use by the other circuits. The timer circuit 909 times the dialing and other hookswitch signals to. determine when the sequence The timeruses the cores in columns H, K, and L to register the time intervals on a binary basis, separately for each link. The sequence circuit 1000 registers the sequence states of the links, which are: normal, tens dialing, units dialing, ringing, busy test, and conversation. The sequence circuit uses the cores in columns M, N, and P to register these states on a binary basis for each link. The dialing supervisory circuit 110%} isprovided to insure that the line circuits are not seized by more than one of the idle links when they become busy, and to prevent idle line circuits from being represented as being busy during scanning. This circuit uses the cores in column B. The line-number-ad vance circuit 12%!) supplies advance or rewrite signals to the line number register 13% to control the number registration in the high-speed memory 150.

i GENERAL INTERCONNECTION DIAGRAM (FIGS. 2-6) FIGS. 26, inclusive, when arranged together as shown in FIG. 13, comprise a diagram showing the interconnections of all the units of the system.

Substations (FIG. 2)

Two of the substations S11 and S22 associated with the line circuits LCll and LC22, respectively, are shown in FIG. 2. The substation S22 is only shown in block diagram form and only the dialing or signaling circuitry of the substation S11 is shown. The substations are all of the same type, and the speech circuitry which is not shown may be of the conventional type. The dialing mechanism comprises as its principle components a multivibrator 259, a switch transistor 278 connected in series with the line and under control of the multivibrator 259, a variable capacitor 275 under the control of ten pushbuttons (not shown) and a switch 269 under control of the same pushbuttons.

The dialing mechanism is designed to provide variablewidth direct-current supervisory signals by interrupting the normal flow of direct-current over the loop circuit for predetermined intervals of time depending on the value of capacitance included in the circuit by operation of one of the pushbuttons. The variation in the voltage drop across a resistor, for example, the resistor 24%, i.e., the point 241 of the line circuit LC11, due to the interruption of the normal flow of direct-current by the dialing mechanism provides the hookswitch and dial pulse signals.

Line Circuits (FIG. 2)

Two of the line circuits are shown in FIG. 2, with one, LCil, shown by a schematic and functional block diagram, and the other, LCZZ, shown as a single block. These line circuits are connected by subscriber lines to their respective stations S11 and S22, in multiple to the multiplex transmission line MLl, to conductors of the control line 134, and in multiple to the line supervision conductors 121. Referring to the connections to the control line 134, the line number pulses are supplied to line circuit LCil on conductors Ti and U1, and to circuit LCZZ on conductors T2 and U2; while for ringing control both line circuits are connected to conductors RG and RDA.

Each line circuit may be considered in two parts, one part relates principally to signaling and supervision circuits, and includes the circuits connected to the subscriber side of transformer are, along with gates 234,236, 236a, 237a, and 233; and the other part relating principally to the audio to multiplex coupling comprises the circuit connected between transformer 21% and the multiplex line ML] along with gate 232. All of the gates 232, 234, 236, 235a, 237a, and 238 are controlled by signals from the line number register (FIG. 5} so that they may be activated to produce output signals in the true condition only when there is coincidence on the two line number conductors, which, for line circuit LCM, are T1 and U1. Only one line circuit may be so activated during each two-microsecond time slot.

It has become conventional in electronic telephone systems to supply direct-currentfor the station through resistance devices coupled to the subscriber line in the line circuit, and to use the variation in voltage drop in this resistance for hookswitch supervision. Here, the negative terminal of the direct-current source is connected through resistors 243 and 24d and a winding section of transformer 216 to one off-die subscriber line wires, and the grounded positive terminal is connected through resistors 247'and 243 on another Winding section of the transformer to the other 'line Wire. To bypass the resistors and direct-current source, a capacitor 242 having low-impedance at voice frequencies is connected between the two winding sections of the transformer. Conductors 253 and 254 are connected from junction points 24-1 and status 245, respectively, of the resistors to control the hookswitch supervision gate 236. The conductor 254 is connected to the negative supply side and therefore is normally in a true condition except during ringing. The signal on conductor 253 follows the hookswitch and dial pulse signals to permit pulses to be transmitted to lead B when the line loop is closed, and to interrupt this train of pulses when the line loop opens.

When the line circuit is in use the first pulse to appear on lead E is supplied to gate 234 via a two-microsecond delay line, the gate 236a and a device 234a, which may be, for example, a monostable multivibrator, designed to provide an output signal for at least a minimum of 80 microseconds, or one transmission cycle. This arrangement assures that the line circuit will not be seized by more than one idle link which is scanning. The gate 234 is enabled to supply a train of pulses to lead C to mark the line busy. The gate 237a is enabled by pulses appearing on the lead B, as well as the line gate multiplex pulses T and U, and assures that output signals will appear on the lead C during dialing when no signal pulses appear on lead E.

To ring the substation S11, coincident pulses are applied to leads RG, T1 and U1 to enable gate 238 and apply pulses over its output lead 255 to the ringing control unit 240. An interrupter voltage is applied over lead RDA. The ringing control unit 2% responds to these signals to apply a ringing signal to the points 245 and 246 of the direct-current feeding resistance network, causing a current to flow over the subscriber line to operate a tone ringer (not shown) of the substation S11. The current flow during the ringing signal causes a signal on lead 253 which looks like an oil-hook supervisory signal. A ground potential is applied to the lead 254 to block the gate 236 during this ringing interval. However, the interrupter voltage on lead RDA is on for two seconds and off for four seconds, and during the oil period the potential on lead 254 returns to the normal negative value so that gate 236 may respond to the hookswitch signal when the subscriber answers.

Voice frequency signals at the transformer 210 are coupled through an impedance converter 212 and a filter which includes inductor 216 to a multiplex transmission gate TGl which is controlled by pulses from gate 232. The transmission gates of all of the line circuits are connected in common to the end MLl of the multiplex transmission line.

Link and Transmission Control Circuits (FIG. 3)

Each of the link circuits includes only the circuitry required for multiplex transmission; the register, line finding, dialing, and other switching control circuitry shown in FIGS. 4 and 5 being shared on a time division basis by all of the links, with storage in the high-speed memory 1541. One of the link circuits LKl, is shown in FIG. 3. It comprises two multiplex transmisison gates T62 and T63, for multiplex connections to the calling and called lines, respectively. These gates are coupled at voice frequency through inductors 314 and 316, with direct-current bias supplied through an inductor 318. The calling line transmission gate TG2 is controlled by pulses on lead DPl supplied through gate 320; and the called line transmission gate is controlled by pulses on DP2 supplied through gate 322, when coincident pulses are supplied over the switch-through conductor S-T. These gate circuits 326 and 322 are of a special type which produce inverted output pulses when all three inputs are true.

A transmission control unit 116, which is inserted in the multiplex line MLl-MLZ includes a clamp circuit 363, a sawtooth generator 354%, a trigger circuit 365, and a flipfiop 366. The sawtooth generator and the trigger circuit are connected respectively by transformers 361 and 362 in series with the common highway conductor TIA-T12. The clamp circuit 353 is provided to reduce cross talk. The sawtooth generator 364, trigger 365, and flip-flop 366 6 along with a control conductor T-C extending from th flip-flop 366 in multiple to all of the transmission gates in the line and link circuits are used in controlling the multiplex pulses in each time slot, as explained in the Faulkner et al., Patent 3,015,699.

Signaling Circuits (FIG. 3)

The signaling unit includes three signaling gates 8G1, 862, and 8G3, for supplying supervisory tones over the multiplex highway to the calling lines. Each of these signaling gates is essentially similar to one-half of a link circuit, in that it includes a transmission gate (not shown) similar to the gate TGZ, an inverting AND gate (not shown) similar to the gate 329 for supplying pulses to the transmission gate, and a network (not shown) for coupling the input tone and direct-current bias to the transmission gate. In each case, one of the inputs to the AND gate is the transmission control lead T-C and the other input is the respective one of the leads B-T, D-T, or R-T supplied over line 162 from the signal state register 14% (FIG. 5). The outputs from the signal gates are connected in multiple to the end TLZ of the common highway. A dial and busy tone generator 34?. supplies busy tone over conductor 345 to signal gate 8G1, and dial tone over lead 346 to signal gate SGZ. The ringback tone is supplied to signal gate SG3 over conductor 347 from a ringback tone generator 344.

The signaling unit 170 also includes a ringing interrupter 34% for supplying interrupter voltage to tne line circuits. The output is supplied for two seconds to each of the leads RDA, RDB, and BBC in turn, so that each is on for two seconds and oil for four seconds during each six-second period. Each of these output leads is connected to approximately one-third of the line circuits. One of the interrupter output leads is also connected to the ringback tone generator 344, to interrupt its output in the same manner as the ringing signals.

Register Control Circuits (FIG. 4)

The register control circuits 4% shown in FIG. 4 receive hooirswitch and line-busy signals on leads E and C, respectively, from the line circuits and supply output signals on conductors 128 from the line number advance unit 12% to control the rewrite, advance, and clearing of th tens and units digits designating the line registered in the line number register 139 (FIG. 5) Clock pulses are supplied to the circuits in FIG. 4 by conductor group 182 from the sources shown in FIG. 6. As may be seen by reference to 'PTGS. 7-9, inclusive, the register control circuits includes logic gates, flip-flops, and arnplifiers. In each circuit, theinput is supplied to the logic gates, and the output is obtained directly from the flip-flops or from output amplifiers. The flip-flop output leads are designated by reference characters either primed or not primed indicating a false or 0 condition or a true or 1 condition, respectively. The output conductors from all of the units except the line number advance unit 12% are shown as comprising a conductor group 461; and these conductors are connected as inputs to several circuits of FIG. 4 and also to the signal state register 14% in FIG. 5, as shown. The timer 9%, sequence circuit 18%, dialing supervision circuit 1160 and line number advance circuit 1299 are also connected by lines 144, 146, 148, and 145, respec tively, to the high-speed memory 159 (FIG. 6).

The timer 9% may advance through eight steps, making one step each 2 microseconds with output states desig- 7 Output conductors are only necessary for three of these states, namely, conductors T F and S2, in sub-group 991 of group 461.

The sequence circuit 1660 has an output conductor H for the normal state, and conductors H H H H and H for successive states, these output conductors comprising sub-group 1061 of group 4&1.

The output of the dialing supervision circuit 11% comprises conductors B and B, in sub-group 1101 of group 401.

The output conductors 128 from the line number advance circuit 124% comprise conductors TA and UA' for advancing the tens register and units register (FIG. 5) respectively, and conductors TB and U3 to the two registers, respectively, for causing rewrite of the same digit in each.

High-Speed Registers (FIG. 5)

Referring to FIG. 5, the line number register 130 comprises a tens register 13% and a units register 13%. The tens register has the five conductor pairs TC to T6 in group 152, and the units register has a similar five pairs of conductors UC to UG in group 154, which are threaded through the columns of the cores in the high-speed memory 15%, as shown in FIG. 1. In each of the registers, the five conductor pairs are associated with live flip-flops for the two-out-of-five number registration. The tens register is supplied with input signals on leads TA and TB for controlling the advance and rewrite, and similarly the units register is supplied with input signals UA' and U3. In the tens register, the two-out-of-five code is translated to a one-out-of-ten code and supplied through individual output amplifiers to the conductors T1 to T0 in conductor group 134, and similarly the units register supplies signals to the leads U1 to US, also in conductor group 134. In addition, a connection is made from the flip-flop output conductors UC-l and UF-l into cable 461 for controlling the line number advance circuit 1200 in FIG. 4.

The signal state register 1460 has five conductor pairs, BT, DT, RG, RT, and ST, in group 156 to columns of the high-speed memory 150 (FIG. 6), each conductor pair being associated with a signal state flip-flop in the register. Input is supplied by conductors in group set from the register control circuits 4%. Output is supplied through individual output amplifiers to leads R-G, R-T, D-T, B-T, and ST. The signals R-T, D-T, and B-T supply control pulses to the signal gates in the signaling circuit 176 (FIG. 3). The signal on conductor ST controls the switchthrough of the transmission gates in the link circuits. The output conductor R-G is shown connected through line 162, and the signaling circuits 179 in FIG. 3 to line control group 134 to the line circuit, to control the ringing of called lines. Both the line number register 130 and the signal state register 14% are supplied with clock pulses over conductor group 184 from the sources shown in FIG. 6.

Pulse Sources and Definitions" (FIG. 6)

The pulse sources 66% are shown in FIG. 6. The primary source is a high-speed clock 619. The output pulses from this clock drive a distributor 612 for driving the high-speed memory 150 and supplies pulses for the transmission circuit. The output of the highspeed clock 638 also drives a four-stage distributor 616 for controlling logic circuits in the register control circuit 4%.

In reference to the pulses, the following definitions relate to the terms used in this application.

Time slot: A two-microsecondinterval, being one complete cycle of-the high-speed clock lll. Each time slot comprises a (LS-microsecond guard interval followed by a 1.5-microseconds interval during which transmission and various'control operations take place.

Transmission cycle: A time interval comprising 40 time slots or 80 microseconds, being one cycle of the distributor 612.

Timer step: An interval of 2 microseconds.

Coincident: Used with reference to two or more signals which overlap in time, usually at the input of a gate.

Simultaneous: Used with reference to signals or events occurring during the same time cycle, such as a trans mission cycle, although possibly in dilierent time divisions of the cycle.

FIG. 12 is a graph of the pulses produced during each time slot by the high-speed clock 610. The pulses on lead CR5 occur during the guard interval and have a duration of 0.5-microsecond. The pulses on lead CF15 occur during the remainder of the time slot and have a duration of 1.5-microseconds. The pulses on lead CPlA and CPlB each have a duration of 1 microsecond and occur during each time slot as shown.

The distributor 612 has forty stages and is driven one stage per time slot. The input is supplied by the pulses on leads CR5 and CF15 from the high-speed clock 610. Each stage drives a row of the high-speed memory 159, and has two output leads threaded through the cores'of the corresponding row. One of the outputs of each stage is a 0.5-rnicrosecond pulse for applying a readout potential to the cores. As shown in FIG. 1, each of these leads is connected to ground on the right hand side of the memory 159. The otheroutput from each stage is a 1.5-microseconds si nal for supplying a /2-write potential to the cores. The leads from these outputs extend through cores of the high-speed memory 15% to the distributor pulse leads DPI to DP46, which are connected individually to transmission gates of the links. Lead DPl is connected to the calling side transmission gate, and lead DPZ is connected-to the called side transmission gate of link 1. The succeeding pairs of the leads DP are connected to succeeding links, each odd-numbered distribu tor pulse being supplied to a calling side gate, and each even numbered distributor pulse being supplied to a called side gate of a link. Thus, each of the forty distributor pulses corresponding to one time channel of the multiplex transmission, and is permanently associated with a link transmission gate.

The distributor 616 has four stages and is driven two stages per time slot. The input is supplied by the pulses on the leads CR5 and CF15 from the high-speed clock 619. The output pulses on the leads A and A and the output on the leads A and A correspond to the CBS and CF15 pulses in the high-speed clock 61% respectively, and are supplied to the various circuits in the register control circuit 499 coincidence with these pulses. Thus, it

may be observed that the pulses on the leads A and A correspond and coincide with the read and /2-write pulses,

respectively, applied to the calling party rows of the highspeed memory 150, and the pulses on the leads A and A correspond and coincide with the read and /z-write pulses, respectively, supplied to the called party rows of the high-speed memory 150.

TIME DIVISION DISTRIBUTION PLAN In the system disclosed in the Faulkner et :11. Patent 3,015,699 there are two groups of time division multiplex r the other group being associated with a low-speed memory. The present system also utilizes two groups of time division multiplex circuits, however, both groups have the same distribution cycle, and both groups are associated with a single high-speed memory 150. One of the groups of circuits controls the time division transmission of voice and tone signals over the multiplex line MLl- ML2. Referring to FIG. 1, and also to FIGS. 2, 3, 5, and 6, this group includes the high-speed clock 610, the highspeed magnetic distributor 612, one portion of the highspeed memory 15%, the line number register 13% the si nal state register 14%, all of the line circuits LCll to LCM, all of the link circuits LKI to LKZG, the signaling circuit-s 170, and the transmission control unit 110. The

high-speed clock 610 has a cycle of two-microseconds which is referred to as a time slot. As shown in FIG. 12, the time slot is divided into (LS-microsecond and 1.5- microseconds intervals, corresponding to the respective outputs CR and CF from the clock. The output pulses from the clock 610 drive the high-speed magnetic distributor 612. This distributor has forty stages of twomicroseconds each, making a total cycle of eighty microseconds. Each cycle of this distributor is one transmission cycle on the multiplex line ML1ML2, and each stage occurring in successive cycles comprises one transmission channel. In each channel transmission occurs during the 1.5-microseconds interval, and the 0.5-microsecond interval is used as a guard interval between channels. The distributor 612 has two output leads for each stage, and for each stage the pair of output leads are threaded through one horizontal row of the high-speed memory 150. On one of the leads a read pulse is delivered during the 0.5-microsecond interval, and on the other a /2-write pulse is delivered during the 1.5-microseconds interval. The writing leads extend through the memory and thence to the links to form the principal distributor pulse output leads. These forty DP leads are grouped in successive pairs extending to the twenty links. For each link the odd numbered DP pulse is used for controlling the calling line gate, and the even numbered pulse is used for controlling the called line gate. Thus, each link uses two adjacent channels in the transmission cycle for a connection between two lines.

The line number register 13% in conjunction with the associated cores in the high-speed memory 150 delivers pulses to the line circuits in coincidence with the pulses delivered to the link transmission gates with which they have been selectively connected.

In accordance with stored information in the highspeed memory 15%, the line number register 13% delivers pulses to the line circuits, and the signal state register delivers pulses to the signaling circuit 170 and to the link circuits LKJ; to LKZO, so that for each channel for which a connection has been established, two transmission gates connected to the multiplex line, one at the end ML} and the other at the end M12, are pulsed in coincidence.

The other group of circuits provide for time division sharing of the circuits used in perforimng most of the logical operations required by the links to set up connections between lines. Referring to FIGS. 1, 4, and 6, these circuits comprise the high-speed clock 619, the magnetic distributor 616, the other portion of the high-speed memory 159, and the register control circuits 4%.

The distributor 616 driven by the high-speed clock 618' produces four output pulses A -A requiring a total of two time slots or four nucroseconds. During the pulse interval A information is transferred from the calling party row of the high-speed memory 156 to the register control circuit 4%; during the pulse interval A the line supervision leads C and E are analyzed, the information obtained from the line circuits and memory is used to perform logical operations, to deliver appropriate output signals on the conductors 128 to the line number register 130 as well as over the line 461 to the signal state register 1460, and the information, which may or may not have been altered, is transferred back into the same row of the memory 15a; during the pulse intervals A and A the called party row is read out, analyzed, and transferred back to the same row of the memory 15 in much the same manner as the calling line rows. Thus, each link shares the register control circuits 4-69 for two time slots.

DETAILED DESCRIPTION The structure and function of the dial mechanism at each of the associated substations will now be described by reference to the dial mechanism at substation S11, shown in FIG. 2. a

The transmitting and receiving portion of the substation has been eliminated for clarit only the dialing it) mechanism being shown. The dialing mechanism is de'- signed to operate with electronic exchanges that use multiplexed pulses to perform logic operations, and particular 1y designed for use in conjunction with the present disclosed electronic time division multiplex communication system.

The dialing mechanism comprises as its principal components a multivibrator 26% including transistors 261 and 265, a switch transistor 278 connected in series with the line and under control of the multivibrator 260, a Variable capacitor 275 under control of pushbuttons (not shown), and a break switch 269 under control of the pushbuttons. There are ten values of capacitance under control of ten pushbuttous designated from 0 to 9, pressing one of the pushbuttons includes one of the values of capacitance represented by the variable capacitor 275 in the multivibrator 269 circuit. These same pushbuttons when pressed operate the switch 269 to open the contact 27 6 to turn on the multivibrator.

The operation of the dialing mechanism may be explained as follows: when the handset (not shown) is removed from the cradle the hookswitch represented by the contact 259 closes the loop circuit between the substathe CR5 and the CPLS pulses have not been shown. in its operated position).

Normally, the switch 269 is not operated and the contact 270 is closed. With contact 270 closed, the base electrode 264 of transistor 261 is held at the same potential as its emitter electrode 263, thus transistor 261 is held non-conductive. A negative potential will be applied through resistor 274 to the base electrode 258 of transistor 265, thus transistor 265 will be conductive. A negative potential also appears at the base electrode 279 of transistor 278 and transistor 278 will also be conductive.

Under these conditions the impedance path through resistor 277, emitter electrode 289 and collector electrode 281 of transistor 278 and resistor 282 (resistor 282 represents the D.C. impedance of the substation) is much less than the impedance through resistor 271 and the emitter electrode 267 and collector electrode 266 of transistor 265, and most of the current flow will follow this path. The above-mentioned loop circuit may thus be traced as follows: from negative battery in the central office line circuit L011, resistors 243 and 244, one winding of transformer 21%, over the line through the hookswitch 259, resistor 277, emitter electrode 280 and collector electrode 281, resistor 232, over the line to the line circuit LCM, through another winding of transformer 210, resistors 248 and 247, to ground. The potential at point 241 will be at approximately 5 volts at this time. If line gate multiplex pulses are present on leads T1 and U1, a 2 volts signal will appear at the output of gate 236 on lead E. If the voltage at point 2 51 comes more positive than 3 volts, there will be no output signal from the gate 236, due to the Zener diode 253a. The signal on the E lead indicates to the register control circuit 4th? that the substation is requesting service, as will be explained.

When the subscriber at the substation presses a pushbutton (not shown), it will first place a capacitance 275 in the circuit, and then cause switch 269 to operate opening contact 279. When the contact 270 opens, transistor 261 will be rendered conductive, shorting the base electrode 279 of transistor 278 across resistor 282 and causing transistor 278 to be non-conductive. Capacitance 275 starts to charge up, drawing electrons from the base electrode 268 of transistor 265 and thus transistor 265 is rendered non-conductive. Transistor 265 will remain non-conductive until capacitance 275 is charged, at which time transistor 265 is again rendered conductive. When transistor 265 becomes conductive again, capacitor 272 will charge up, drawing electrons from the base electrode 264 of transistor 261, thus rendering transistor 261 nonconductive and transistor 278 conductive. The multivibrator 260 will continue to oscillate as long as switch 269 is operated holding contact 276 open. When the contact 270 is closed (the pushbutton is released) transistor 261 is held non-conductive and oscillations cease. The length of time that the multivibrator 260 remains in its second state of operation, i.e., that transistor 261 is conductive, depends on the value of capacitance 275 included in the circuit by pressing one of the ten pushbuttons. The value of capacitance 275 must be such that transistor 265 (during the time contact 270 is open) remains on for a time that is less than the interdigital pause, which for the present disclosure is equal to at least eight transmission cycles allowing the timer to reach state 9. The significance of this will be explained in the disclosure which follows.

During the time that transistor 273, is non-conductive the circuit through resistor 282 is open and the only current path is through the high-impedance path including the emitter electrode 263 and collector electrode 262 of transistor 261 and the resistor 276. Under these conditions the normal flow of direct-current will be much less and the voltage at the point 241 will be less than -3 volts. With the voltage something less than 3 volts, the Zener diode 253a will not conduct and there will be an absence of pulses on the E lead; The normal flow of direct-current over the line is therefore interrupted at a high-speed to provide variable-width direct-current supervisory signals to the register control circuits40ii.

The table below shows the relationship between the sample time and the direct-current signal pulse width. In the table, T C=one transmission cycle.

Signal Width Signal No.

1 TC X1 2TC z 3TC X2 4TC 3 src x erc 4 7TC X4 8TC 5 9TC X5 IOTC 6 11TC XG 12TC 7 l3TC X l4TC s 15TC X8 16TC 9 l7TC X 18TC l9TC X 2OTC The structure and function of the switching control circuits will now be described with reference to individual functional block diagrams.

The logic circuitry is direct-coupled (DC), that is, signals are represented by steady-state voltages. Two levels are employed. The first-level is usually volts,

although other negative voltages are used in a fewplaces,

cuits are included in each unit for controlling the'setting of the flip-flops, which may be set 1 or set 0. Set when used alone set 1, and reset is synonomous with set 0. Also, an unprirned character represents a true or 1 condition and'a primed character represents a false or 0 condition. These logic circuits comprise diode type AND and OR gates. I

Boolean Algebra Iudescribing the-logical operations performedby the circuits, Boolean algebra equationsare used. In this notation the addition symbol signifies GR, and the multiplication symbol, expressed or implied, signifies AND,

and the prime symbol signifies the inverted condition.

7 Register Control Circuits The: register control circuits 4% shown iHiFIGS 1 and 4 Will now be described with reference to the functional block diagrams of FIGS. 7-9, and the logical operations performed by these circuits as individual units will be explained. Reference may also be made to the graph shown in FIGS. 10 and ll'which shows the operation of the various units during the scanning, seizure, dialing, and the establishing of a connection in a call.

In referring to the graph shown in FIGS. 10 and 11, it may be noted that only the time interval associated with one link during each transmission cycle is shown. The operation of each of the other links is similar and the operation of all ofthe links may be understood by observing only the one link, and the disclosure is greatly simplified. In addition, the pulses Ai-A, are shown as pulses of equal duration rather than as pulses of different duration, i.e., the pulses A and A are equivalent to a CR5 pulse and the pulses A and A are equivalent to a CPLS pulse. Furthermore, to still simplify the disclosure, since the pulses A and A and the pulses A and A occur simultaneous with the CR5 pulse and the CPLS pulses, respectively, applied sequentially to the calling and called party rows of the high-speed memory the CH5 and the CP 1.5 pulses have not been shown. The presence of these pulses is to be understood, however, when reference is made to reading out a core during the pulse interval A or A or writing into a core during the pulse interval A or A In describing the operation of the various units, the operations will be related to particular transmission cycles shown in the graph of FIGS. 10 and 11 by a note to the particular transmission cycle in a bracket, e.g., (TC9).

Timing Circuit The timing'circuit shown in FIG. 7 is essentially a binary counter and includes three flip-flops FF-H, FF-K and FFL and the associated columns of the high-speed memcry 15%. It may be noted that these columns only have cores in the calling party rows, i.e., the odd numbered rows, since the timer is controlled by the calling party. Since these flip flops are shared'on a time division basis by all of the links, each link having its own row of cores in the high-speed memory 159, the timer state of each link is independent from that of the others.

The logic equations for operation of the timing circuit are:

For writing into the memory cores:

For setting'the timer flip-flops:

51:5,, II A K S K'ZA L =A 1 The operation of the timer circuit may be explained as follows: initially ail of the timer flip-flops are set 0 (H, K and L are all true) and the timer is at T The timer flip-flops are set 0 by the lead edge of A and are set '1 by the readout of the memory cores which will come slightly after the lead edge of A The timer flip-flops will be registered for two successive time slots, i.e., the calling and called party time slots.

When E is true during A no cores will be written into. The first E pulse to appear coincident with A for this row (TC'T) will Write into the [1 core. One transmission cycle later, (TC8 during A core 11 will be readout and flip-flop FF-H will be set 1. Since H will now be true during A 12 core cannot be Written into again but it core will be since the equation HKE is true.

A study of the equations will show that at the end of eight successive E pulses (TC7 to TC14), HKL, and thus 9, will be true. As long as E reappears during A Q will be true and the timer will stay in this condition. Once B does not appear (5' true) during A however, the timer will return to T no matter what the state of the timer was at the time.

A more detailed explanation of this timer operation is as follows: as previously stated, initially all of the timer flip-flops are set 0 (H, K and L are all true) and the timer is at T Whenever E is true during A there will be no output from the gates 909, 923 and 939, thus none of the memory cores associated with the timer circuit will be written into.

It may be noted that the gate'905 is arranged to inhibit the gate 997 whenever CT H is true, i.e., whenever CT H is true the output from the gate 905 will inhibit the gate 907 and there will be no output from the gate 997. If CT I-I is not true, however, gate 905 will have no effect on gate 907, and if E and H or Q are true there will be an output from gate 997.

When E first appears coincident with A (TC7) the signal on the E lead is gated through the gates 997 and 909, and core h is Written into. During the next transmission cycle (TCS) the leading edge of the A pulse sets FF-H false. Flip-flop FFH is set 1, however, shortly thereafter when the 11 core is readout. In the explanation which follows this operation, i.e., the flip-flops being set 0 by the leading edge of A and set 1 shortly thereafter by the readout of the core associated with the flipfiop, will be assumed and not restated each time. Also,

-on the graph shown in FIGS. 10 and 11 the state of the flip-flops are shown as continuous rather than trying to differentiate between these two operations since the time interval is very short.

During the A pulse of this transmission cycle (TCs) core h cannot be written into since H is now true and the input to gate 993 is false. The k core will be written into, however, since HK' is true at the input to gate 917 and EA is true at the input to gate 921. The k core is readout and the flip-flop FF-K is set 1 during the next A pulse (TC9). Also, the flip-flop FF-H is set 0 during this A pulse.

The succeeding A pulse, since H is true at the input to gate 993 and HK is true at the input to gate 917 the signal on the E lead is gate through the associated gates and both the h core and the k core are written into. During the A pulse of the following transmission cycle (TCiG) both flip-flop FF-H and FF-K are set 1 when the h and k core are readout. Also, since H and K are now both true during A the input to gates 933 and 935 will be true (HKL true) and the inputs to the gates 937 and 939 will all be true and the 1 core will be written into. Flip-flops FFH and FF-K will be set 0 and the flip-flop FF-L will be set 1 during the following A pulse (TCll).

During the A pulse (TCll) the h core will be written into again and also the 1 core since H is true at gate 903 and at gate 929. The k core is not written into since neither HK or H'K is true at the input to gates 915 and When the h and 1 cores are readout during A (TC12) flip-flops H and L are again set 1, and during A the k and 1 cores are written into but the h core is not (HK' is true at gate 915 and K is true at gate 929). During the A pulse (TC13) the cores k and l are again readout to set flip-flops FF-K and FFL true. During the following A pulse all of the cores h, k, and I will be written into since now H is true at gate 903, H'K is true at the gate 917, and H is true at the gate 929.

It may also be noted that at this time (TC13) when H'KL is true that the timer state will advance to F. The significance of this timer state will be explained in relation to the sequence circuit during the discussion of the sequence circuit.

' true, and the sequence state advances from 11,, to H During the next A pulse (TC14) all of the flip-flops FF-H, FF-K and FF-L will be set 1 when the h, k and l cores are readout. With HKL true the timer advances to 9. Each of the cores h, k and I will be written into during each succeeding A pulse as long as E continues to appear during A since the input to gates 903, 919 and 935 will be true as long as Q is true. And Q will be true as long as E remains true.

Sequence Circuit The sequence circuit shown in FIG. 8 includes three flip-flops FF-M, FFN and FF-P and the associated columns of the high-speed memory 150. These flip-flops also are shared on a time division basis by all'of the links and the sequence state of each link is independent from that of the others.

It may be noted that the columns associated with the sequence circuit only have cores in the calling party rows, i.e., the odd numbered rows. Furthermore, these cores are threaded by the read winding associated with the calling party row but rather than being threaded with the /2-write winding associated with the calling party row they are threaded with the /2-write winding associated with the called party rows, i.e., the even numbered rows. This allows the information stored in these cores to be read out to the register control circuits with the calling party read pulse, analyzed and altered if necessary, and to be rewritten into these cores during the called party /z-write pulse, thus the flip-flops associated with these columns are set for 2 time slots.

The logic equations for the sequence circuit are:

For writing into the memory cores:

For setting the sequence circuit flip-flops:

Sequence states:

H =MN'P (normal) H =MN'P (seizure or tens advance) H =MNP (units advance) I =MNP (ringing) H =M 'N 'P (conversation) It may be noted by reference to FIG. 8, the logic equations and to the graph shown in FIGS. 10 and 11 that the sequence state can be advanced only when the timer state reaches 9 (TC14). The timer can reach S2 only when E is true during A for 8 consecutive transmission cycles, and this condition occurs only after seizure or during the interdigital pause, this will be subsequently explained.

When the timer reaches state F (TC13), aspreviously explained, during the A, pulse of that transmission cycle core m will be written into since the input to gates 1961 and 1693 will both be true. The following transmission cycle (TC14) during A flip-flop FF-M is set It will be observed that once a core is written into it will continue to rewrite every transmission cycle unless the flip-flop is reset after readout and before write-in.

The conditions when this occurs is shown in the equations. l i

During this same transmission cycle (TC14) the timer advances to state S2, as previously explained, and will remain at 9 until E becomes true during A When the sequence circuit has advanced to H the called partys number may be dialed. The subscriber at the substation presses any pushbutton and the loop circuit will be effectively opened and closed as long as the pushbutton is pressed, as previously explained. The period of time that theloop circuit is elfectively open will depend on the pushbutton pressed. Since the period between the pulses, i.e., the time that the multivibrator remains in its second state effectively closing the loop circuit, is set and will never be greater than six transmission cycles the timer cannot reach 9 and the sequence circuit will not advance.

When the subscriber has dialed the first digit (the registration of the digit dialed will be subsequently explained during the description of the operation of the line number advance circuit) and releases the pushbutton the loop circuit is again effectively closed and will remain closed for at least eight consecutive frames. It was found during numerous experiments with the pushbutton dial that it was practically impossible for a subscriber to release one pushbutton and press another one in an interval of time less than the required eight transmission cycles which is used to indicate an interdigital pause.

if E is true for eight consecutive transmission cycles (TC19 to TCZ) the timer will again advance through F to Q. When the timer reached F (TCZS) and M is true the :1 core is written into during the A, pulse, since the input to gates 1913, 18115, and 1017 will all be true. After the timer reaches Q (TC26) the next transmission cycle (TC27) then core is read-out during A and the fliplop FF-N is set true. Flip-flops FF-M and FF-N will both now be true and the sequence state will advance to H The subscriber may now dial the second digit, the operation of the timer and sequence circuit being essentially the same.

When the subscriber releases the pushbutton after the second digit is :dialed the timer again advances through F to S2 (TC32 to TC39). When the timer reaches F (TC38) the p core will be written into during A since the input to gates 1027, 1629 and 1031 will all be true, and flip-flop FF-P will be set true during the following A pulse (TC39) when the core is readout. vVhen P is true along with M and N the sequence state advances to H If the called line is busy (the conditions existing if the called line is busy is shown during TC40 to TC42) E and C will both be true during A and flip-flop FF-M will be set since the input to gates 1%9 and'1067 will be true. When 1 becomes true the sequence state will advance to H (TC) and a busy signal will be transmitted to the calling subscriber. When the calling party hangsup and C becomes true during A flip-flops FF-M, FF-N and FF-P will all be set false since the inputs to gates 1011, 10.97, 1925, 1321, 1937 and 1035 will all be true, and the sequence state will advance to H (TCdZ).

If the called line is not busy the sequence will remain in H (TC43), signal the called partly and mark his line as busyduring ringing, C true. When the called party answers E will become true during A (TOM) and flip-flops FF-N and FF-M will be set'O since the input to gates 1009, 1067, 1023and 1021 will all be true. The sequence state advances to H (TC4G) and the connection will be complete.

When the calling party hangs-up the busy mark will no 1 longer be held and C will no longer be true. When C becomes true flip-flops FF-M, FF-N and FF-P will all be set 0 during A since theinputs to gates 1011 and 1067,

. 1025 and1021, and 1037 and 1935 will all be true, and the sequence state will advance to H (T047).

Busy Circuits The busy circuit is shown in FIG. 9 and includes the memory 150. The flip-flop is also shared on a time diviflip-flop FF-B and the associated column of the high-speed 16 sion basis by each of the links, and the state of each link is independent from that of the other.

A busy mark must appear on each line circuit within two microseconds and be such that one input pulse will hold it on for a minimum of microseconds. As previously stated, when the line circuit is in use the first pulse to appear on lead E is supplied to the gate 234 via a two microsecond delay line, the gate 236a, and a pulse stretching device 234a, which may be, for example, a monostable multivibrator, designed to provide an output signal for at least a minimum of 80 microseconds, or one transmission cycle. The first pulse appearing on the lead E must be delayed for two microseconds to allow the h core associated with the flip-flop FF-H of the timer circuit to be written into. If the pulse was not delayed and the pulse appeared on lead C during the same interval H T C would be true at the input to gate 905 and the gate 907 'would be inhibited preventing the h core from being written into. a

The busy signal C must, however, appear true within two microseconds after E first becomes true to prevent the line from being seized by more than one idle link which is scanning.

Furthermore, once the line is seized by an idle link the busy mark must be provided during dialing. Since E will be true during dialing, and may be true for longer than 80 microseconds, further provisions must be made for maintaining C true during dialing to prevent it from being seized by another calling line. The busy marking flip-flop FF-B prevents this from happening. The signal B will become true one transmission cycle after E becomes true and will remain true during each time interval associated with this calling link. The opertaion of the busy circuit may be better understood by observing the logic equations which are:

For writing into the b core:

2+( 3 4 For setting the flip-flop FF-B:

Referring now to FIGS. 2 and 9 the operation of the busy circuit will be explained. When the line circuit is in use the first pulse to appear on lead E will be supplied to the gate 234, after a two microsecond delay, as previously explained. In addition, since E is true during A; the input to gates 1101, 1103, and 1105 will all be true and the ,b core will be written into.. The following transmission cycle the 11 core will be readout during A and the flipflop FF-B will .be set true. The inputs to gate 237a will all be true when the line circuit LC11 is scanned, T U B are all true, and the output is supplied through the gate 236:: to the pulse stretching device 234a to again set its output true for another 80 microseconds. The output of the device 234a is supplied to the gate 234 and since T U is true, C will betrue at the output of gate 234. If E is still true the device will also be set after the two microsecond delay from thesignal on the lead E.

It may be noted that once B becomes true the core b is written into during A and the flip-flop FF-B will be set true during each cycle until it is reset. If now E becomes true during dialing the pulse stretching device 234a will be set during each cycle since the inputs to gate 237a will be true and will provide an output to the gate 234.

If the sequence advances to H (busy test and ringing) and the called partys line circuit is idle it must be marked busy during ringing to prevent it from being seized by a second calling line. When the sequence advances to H during A the b core of the called partys memory row 1? true and the pulse stretching device similar to the pulse stretching device 234a will be set and the busy signal C will be true.

Line Number Advance Circuit The line number advance circuit shown in FIG. 9 includes two flip-flops FF-X and FF-Y and the associated columns of the high-speed memory 150. These flip-flops like the flip-flops in the timer circuit and the sequence circuit are also shared on a time division basis by all of the links and the line number advance of each link is independent from that of the others.

The line number advance circuit determines whether the tens register 1300 and the units register 1302 in the line number register 13% will advance, rewrite, or clear for a link during its logic cycle. "The line number register is controlled for both line finder action of an allotted link scanning for a calling line, and connector action of registering 2. called number during dialing. Since during a links logic cycle, A 'coincides with the calling line and A coincides with the called line, those gates in FIG. 9 which relate to line finder action have an A input, and those which relate to connector action have an A input.

The logic circuits in the line number register 13% are designed to use inverted signals; therefore, the output signals from the line number advance circuit 12110 are also in inverted form. The output amplifiers 1225, 1233, 1235 and 1241, as well as the amplifiers 1229 and 1239, perform the functions of both amplifying signals and inverting them. The signals TA and UA are for tens advance and units advance respectively, and the signal TB and UB are for tens rewrite and units rewrite, respectively. The output is normally in a condition for rewrite, that is, TA and UA are at ground so that TA and UA' are at a negative potential, and TB and UB are at a negative potential so that TB and U13 are at ground.

The logic equations for the line number advance circuits are:

For setting the memory cores:

For setting the line number advance flip-flops:

For tens and units advance:

A scanning link, that is one in which the sequence circuit has the normal output H controls the line finder advance by means of gates 1213 and 1219. The units may advance in any logic cycle, while the tens may ad- Vance only if UC-l and UF-l (code for Uh) from the units register 13131 are true. Advance occurs if the scanned line is on hook or busy since either E or C will be true and the x core will be written into and FFX subsequently will be true at the irrut to gate 1213.

For connector action, gate 1223, produces a tens ad- Vance once each transmission cycle occurring in the time of the first dial pulse during the dialing of the tens digit with the sequence in its H state, and the gate 1215' permits the advance once each transmission cycle occurring in the time of the first dial pulse during the dialing of the units digit with the sequence in its H state.

The tens rewrite signal is inhibited whenever. the tens advance signal appears by applying the inverted'signal from lead TA to gate 124%; and similarly the units rewrite is inhibited by the units advance by connecting the inverted signal from UA to gate 1231. The rewrite may also be inhibited by the connector without an advance signal to clear the register. The connector supplies the clearing signal at gate 1237 when the sequence advances to H before the beginning of the tens digit, with Y and 9 true; and at gate 1227 when the sequence has advanced to 1-1 before the beginning of the units digit, with Y and 9 true. The output of gate 1227 is inverted by amplifier 1229 and applied to AND gate 1231; and the output of gate 1237 is inverted by amplifier 1239 and applied to AND gate 1240.

The detailed operation of the line number advance is as follows: assuming an idle link the sequence state is at H and the timer is at T as previously explained. A line is sampled during A (TC1) and if the line is idle B will be true. input to gates 1201, 12% and 1209 are all true. The following transmission cycle during A (TCZ) core at is readout and the flip-flop FF-X is set 1. During A since X is now true, core x cannot be written into again. At the output of gate 1217 (the units advance gate U and output signal will appear since XHDA2 are all true at the input to gates 1213 and 1217. The signal is inverted by amplifier 1225 to provide U Gate 1231 is now inhibited and the units rewrite signal will be false.

if the units number that had been sampled was U then v the tens number will advance also since the input to gates 1219 and 1221 are also true, the output signal again being inverted by amplifier 1233 to provide T,,.

The next transmission cycle (TC3) X will be true and the advanced number wil be readout of the cores. If number is also idle, core x will again be written into, and the cycle will continue.

If a line is sampled that is busy in some other time slot then the busy mark C on that line will be true. Since C is true the line number will continue to advance even though E is true for the input to gates 12%, 12495 and 1209 will still be true. Also the timer will not ad- Vance if the line sampled is busy since core h cannot be written into when CT l-I is true at the input to gate 905.

Vihen the calling party row is advanced to a number that is waiting to be seized, E will be true and during A (TC7) the 11 core will be written into, as previously explained. I The core x can no longer be written into while the sequence remains at H for E is true. When E has appeared for eight consecutive transmission cycles (TC7 to TC14) the timer will have advanced through F to Q.

When the timer reached F (TC13) the core 1 was Writ ten into during A for the input to gates 1243 and 1245 were both true, and during the succeeding A (TCl l) pulse flip-flop Y was set 1. Also, when the timer reached 52 which was at the same time Y became true the sequence state advanced to H Since the sequence circuit will not return to normal until the calling party hangs-up and C is no longer true, the calling party registration cannot.

(TC16) the input to the gates 1203,12il7 and 1299,

will be all true during A and core x willbe written into. One period later (TC17) x core will be readout during A and flip-flop FF-X will be set 1. A tens advance pulse willbe transmitted during A whenthe input to gates 1223 and 1221 are all true. Core x will not be written into during this transmission cycle for X is true. If E has not become true during the next transmission cycle (TC18) core x will again be written into and the Core x will be written into since the 1 

13. IN A TIME DIVISION MULTIPLEX COMMUNICATION SYSTEM OF THE TYPE WHEREIN EACH OF THE SUBSCRIBER SUBSTATIONS ARE INDIVIDUALLY CONNECTED BY A PAIR OF LINE CONDUCTORS TO A SOURCE OF DIRECT-CURRENT IN AN ASSOCIATED LINE CIRCUIT AND WHEREIN COMMON CONTROL EQUIPMENT SHARED ON A TIME DIVISION MULTIPLEX BASIS IS CONTROLLED BY SUPERVISORY SIGNALS EXTENDED FROM SAID SUBSCRIBER SUBSTATIONS FOR EFFECTIVELY ESTABLISHING CONNECTIONS BETWEEN INDIVIDUAL ONES OF SAID SUBSCRIBER SUBSTATIONS IN ACCORDANCE WITH SAID SUPERVISORY SIGNALS, THE COMBINATION THEREWITH INCLUDING A CALLING DEVICE AT EACH OF SAID SUBSCRIBER SUBSTATIONS FOR PROVIDING VARIABLE-WIDTH DIRECT-CURRENT SUPERVISORY SIGNALS INCLUDING A NORMALLY CONDUCTIVE SWITCHING TRANSISTOR CONNECTED IN SERIES WITH SAID PAIR OF LINE CONDUCTORS, A MUL- 